Tracker: GCS_MAVLink now uses Mission singleton

AntennaTracker/GCS_Mavlink.h

@@ -17,7 +17,6 @@ protected:
     // as currently Tracker may brick XBees
     uint32_t telem_delay() const override { return 0; }
 
-    AP_Mission *get_mission() override { return nullptr; };
     AP_Rally *get_rally() const override { return nullptr; };
 
     uint8_t sysid_my_gcs() const override;